Turbofan engines commonly utilize compressor boost stages, or booster, driven off the fan rotor to supercharge the core thereby increasing the overall pressure ratio. Since the compressor boost stages are driven off the low pressure spool of the engine and have a tip diameter set generally by the fan blade hub line, the tip speed of the compressor boost stages is relatively low. This can lead to an engine design utilizing fairly large number of compressor boost stages to achieve a high pressure rise in the booster.
Geared fan arrangements are known where an epicyclic gear train is introduced between the low pressure turbine and the fan, thereby allowing the low pressure turbine to run at a higher speed than the fan. The compressor boost stages are then coupled to the low pressure turbine through the low pressure shaft. This arrangement not only achieves more desirable compressor boost stage (now an intermediate pressure compressor) tip speeds, but it also allows the low pressure turbine to run more efficiently. The geared fan in this arrangement may have some drawbacks. First, the epicyclic gear train is driving the fan rotor and must be designed to take the relatively heavy load of the fan. This creates a design challenge for the gear system where weight, cost, and heat generation must be minimized. Length can also be an issue as the arrangement implemented in many earlier designs introduces an additional support frame between the fan and the intermediate pressure compressor. The second frame not only adds length, but it also adds weight and cost.
Accordingly, there remains a need for further contributions in this area of technology. The present invention provides many novel and nonobvious contributions to this area of technology including a system for driving the compressor boost stages in a gas turbine engine.